Pretty right, we get a rough deal, cannot book with a US T/A on most cruise lines including, Princess, HAL, Cunard, Carnival, Costa, all Carnival Group lines.
Also the prices we have to book through Princess Australia website are more than booking through Princess US site, which we cannot access.
I know of many Aussie cruisers who have booked through the USA with an Aussie t/a..don't know if anyone has ever been denied boarding though. And I don't know how the Aussie t/a does it, must getting around the policy somehow.
Now you know why many Aussies remove their auto tips once on board, they think they have already contributed to the Carnival wage fund by paying big and unfair cruise prices.

I don't want to start a war on the auto tip subject, enough of that on CC. But I leave my thoughts with you.